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[URGENT] fgets reading last line in file twice

 
 
DJP
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Posts: n/a
 
      10-18-2004
Hi,

I need to read a file programmatically until end of file. My logic is as
follows:

while(!feof(Fp))
{

fgets(readLine,10000,Fp);

 
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Vyacheslav Kononenko
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Posts: n/a
 
      10-18-2004
DJP wrote:

> Hi,
>
> I need to read a file programmatically until end of file. My logic is as
> follows:
>
> while(!feof(Fp))
> {
>
> fgets(readLine,10000,Fp);
>
> .
> .
> .
> do something
> .
> .
> .
> }
>
>
> However with this logic, depending on the file I am reading my program
> sometime reads the last line twice. Can anyone please tell me why this is
> happenning?
>
> This is kind of urgent so your speedy help will be greatly appreciated.
> Thank you!!
>
>

feof() returns true AFTER you try to read past the end of file.
So this code should work better:
while(true) {
fgets(readLine,10000,Fp);
if( feof(Fp) ) break;
...
}

--
Regards,
Slava

 
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Thomas Matthews
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Posts: n/a
 
      10-18-2004
DJP wrote:
> Hi,
>
> I need to read a file programmatically until end of file. My logic is as
> follows:
>
> while(!feof(Fp))

Here you are testing for EOF before the flag is set.
The flag is set by a read action. See the FAQ below.

> {
>
> fgets(readLine,10000,Fp);
>
> .
> .
> .
> do something
> .
> .
> .
> }
>
>
> However with this logic, depending on the file I am reading my program
> sometime reads the last line twice. Can anyone please tell me why this is
> happenning?
>
> This is kind of urgent so your speedy help will be greatly appreciated.
> Thank you!!
>
>


You'll get faster results by consulting the FAQ first.
I removed all but the C and C++ language newsgroups.


--
Thomas Matthews

C++ newsgroup welcome message:
http://www.slack.net/~shiva/welcome.txt
C++ Faq: http://www.parashift.com/c++-faq-lite
C Faq: http://www.eskimo.com/~scs/c-faq/top.html
alt.comp.lang.learn.c-c++ faq:
http://www.comeaucomputing.com/learn/faq/
Other sites:
http://www.josuttis.com -- C++ STL Library book

 
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John Harrison
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Posts: n/a
 
      10-18-2004

"DJP" <(E-Mail Removed)> wrote in message
news:cl0sc1$g7v$(E-Mail Removed)...
> Hi,
>
> I need to read a file programmatically until end of file. My logic is as
> follows:
>
> while(!feof(Fp))
> {
>
> fgets(readLine,10000,Fp);
>
> .
> .
> .
> do something
> .
> .
> .
> }


Your logic is very common but wrong

while (fgets(readLine,10000,Fp), !feof(fp))
{
do something
}

>
> However with this logic, depending on the file I am reading my program
> sometime reads the last line twice. Can anyone please tell me why this is
> happenning?


Because the return of feof only accurately reflects the status of the
previous read, not the next one.

It sometimes seems that every single newbie in the world gets this wrong, so
you are not alone.

john


 
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John Harrison
Guest
Posts: n/a
 
      10-18-2004

"John Harrison" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
>
> "DJP" <(E-Mail Removed)> wrote in message
> news:cl0sc1$g7v$(E-Mail Removed)...
>> Hi,
>>
>> I need to read a file programmatically until end of file. My logic is as
>> follows:
>>
>> while(!feof(Fp))
>> {
>>
>> fgets(readLine,10000,Fp);
>>
>> .
>> .
>> .
>> do something
>> .
>> .
>> .
>> }

>
> Your logic is very common but wrong
>
> while (fgets(readLine,10000,Fp), !feof(fp))
> {
> do something
> }
>


Actually I don't think that code is correct. My unfamiliarity with C.

Try this

while (fgets(readLine,10000,Fp))
{
}

john


 
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Martin Ambuhl
Guest
Posts: n/a
 
      10-18-2004
DJP wrote:

> Hi,
>
> I need to read a file programmatically until end of file. My logic is as
> follows:


Instead of

> while(!feof(Fp))
> {
> fgets(readLine,10000,Fp);

/* do something */
> }


use
while(fgets(readLine,10000,Fp))
{
/* do something */
}

If this seems off-topic in your newsgroup I apologize to comp.lang.c++,
comp.protocols.tcp-ip, comp.unix.programmer, and comp.unix.solaris, all
of which DJP cross-posted. I have no idea what newsgroups he actually
reads, and so respond to all his listed newsgroups. It is, however,
clear that he has not bothered to check the FAQs on past traffic in any
of these before posting. I hope he corrects his shotgun posting and
corrects his failure to behave like a human being by checking the FAQs
and past traffic before posting.
 
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Maurizio Loreti
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Posts: n/a
 
      10-19-2004
"DJP" <(E-Mail Removed)> writes:

> Hi,
>
> I need to read a file programmatically until end of file. My logic is as
> follows:
>
> while(!feof(Fp))


FAQ. From the comp.lang.c FAQ list,
http://www.eskimo.com/~scs/C-faq/top.html :

12.2: Why does the code

while(!feof(infp)) {
fgets(buf, MAXLINE, infp);
fputs(buf, outfp);
}

copy the last line twice?

A: In C, end-of-file is only indicated *after* an input routine has
tried to read, and failed. (In other words, C's I/O is not like
Pascal's.) Usually, you should just check the return value of
the input routine -- fgets(), for example, returns NULL on end-
of-file. In virtually all cases, there's no need to use feof()
at all.

References: K&R2 Sec. 7.6 p. 164; ISO Sec. 7.9.3, Sec. 7.9.7.1,
Sec. 7.9.10.2; H&S Sec. 15.14 p. 382.

--
Maurizio Loreti http://www.pd.infn.it/~loreti/mlo.html
Dept. of Physics, Univ. of Padova, Italy ROT13: http://www.velocityreviews.com/forums/(E-Mail Removed)
 
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glen herrmannsfeldt
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Posts: n/a
 
      10-21-2004
John Harrison wrote:
> "John Harrison" <(E-Mail Removed)> wrote in message
> news:(E-Mail Removed)...
>
>>"DJP" <(E-Mail Removed)> wrote in message
>>news:cl0sc1$g7v$(E-Mail Removed)...


>>>I need to read a file programmatically until end of file.
>>> My logic is as follows:


>>>while(!feof(Fp))
>>>{
>>> fgets(readLine,10000,Fp);

(snip)
>>Your logic is very common but wrong


>>while (fgets(readLine,10000,Fp), !feof(fp))
>>{


> Actually I don't think that code is correct. My unfamiliarity with C.


> Try this


> while (fgets(readLine,10000,Fp))
> {
> }


As someone else noted, sorry for posting to so many groups.

The reason the latter is preferred is that it also takes
care of the I/O error case, where fgets() returns null,
but EOF has not been reached. An uncorrectable error would
otherwise result in an infinite loop. After the loop one might
test both feof() and ferror(), but most of the time I would
prefer to exit in both cases.

-- glen

 
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